I think there was some bad vibes when they got bought by a less than reputable company a while back. I know a lot of people, myself included switched to Mullvad. I am on Proton now though for the port forwarding.
The most common use case is probably bittorrent. Without port forwarding, you won't be connectable. But anything where someone might need to connect to your local machine from the internet, like hosting game servers or other self-hosting.
You have no problem downloading because your client is initiating the connection. But people won't be able to initiate a connection to you. If you're just leeching off public trackers and don't care about your ratio, then that might not matter to you. But if you're trying to maintain a ratio on a good quality private tracker that's a no go.
You can use a site like this with your VPN ip and the port you have configured for bittorrent while your bittorrent client is up to see if you're connectable.
I'm on Express VPN only because they apparently specialise in avoiding geoblocks and VPN detection for overseas TV sites etc. (Plus three months free for being a TWiT.) So far it's true for BBC iPlayer, RTe Player and UK Channel 4. For this purpose I'm not overly worried about how log-resistant they are, but interesting to keep up with the score here. The integrated 'ad blocking' is also useful, but slower than AdGuard as it seems pages have to wait for assets to fail to load before displaying rather than just being 404'd.
Possibly, or they have multiple entry points on residential ISP blocks and don't have too many people NAT'd per IP so it looks legit. That would explain the higher costs.